Key to the operation of the present invention, as in others mentioned herein, is the fact that sound waves propagate in isotropic or homogeneous materials in such a way that distance covered by the sound wave is linearly proportional to the time required. Thus, if the velocity is known and the return time of an echo is measured, the distance can readily be calculated with an accuracy or resolution, depending upon the measured qualities. Since velocities can be measured within one part in one thousand, 10.sup.-3, and time can be measured in nanoseconds, 10.sup.-9 seconds, or better, thicknesses may be measured to a resolution of 10.sup.-4 inches.
Prior art attempts to measure thicknesses of specimens such as gun barrels and the like are described in U.S. Pat. No. 5,182,139, in the name of J. Frankel and M. Doxbeck issued on Jan. 26, 1993. However, in that system it was not possible to enable the user to continuously or intermittently or by remote command in any time interval measure the thickness changes under conditions in which the temperature of the specimen changes.
The problems with the Frankel and Doxbeck patent were overcome by U.S. Pat. No. 5,557,970 in the name of A. Abbate, et al. issued on Sep. 24, 1996. In U.S. Pat. No. 5,557,970, as the sound wave is transmitted, the time for the sound wave to traverse the sample thickness and the first and second reference thicknesses is measured. Each standard includes a disk that is fluid coupled to a transducer. The thickness of the sample is determined by calculating the velocity of the signal from the traverse times for the first and second reference thicknesses. The calculations are performed by computer and are repeated over a period of time to calculate a change of thickness as a function of time. Simultaneously, thermocouples are attached to the sample being read by a temperature board whose data is fed into a computer. The computer monitors the output of the transducers and thermocouplers, and calculates the temperature compensated thickness or change in thickness as a function of time.
However, the above art does not solve the unique problems arising in the measurements on culverts.
Large culverts made of galvanized steel are used to allow water from a stream or river to flow from one side of a highway to the other. The water that flows through these culverts tends to corrode their interior. After the protective coating (the galvanizing) is gone, the underlying steel starts to corrode, and the wall thickness is reduced. A minimum wall thickness over a given region is needed in order for the culvert to remain capable of sustaining the weight of the earth and traffic above. These culverts have estimated safe lives which are based on rough experience since wall properties can vary. An unforeseen premature failure of a culvert due to corrosion can be the cause of injury or loss of life and at the very least extended traffic disruption. In spite of substantial costs, they are periodically replaced. The premature replacement of a culvert because of incorrectly perceived safety issues, i.e., perceived corrosion and therefore deterioration of wall thickness can present unneeded expenses. Thus an inspection technique with the reliable capability of measuring the wall thickness of culverts can be both a means to save unnecessary replacement expenditures (thus an impetus to preventive maintenance) and a means of preventing traffic catastrophes.
Ultrasonic pulse-echo techniques have been used to measure wall thicknesses to obtain an indication of the safety of culverts. This generally consists of an ultrasonic transducer being placed on a grease coated culvert wall and hooking it up to an electronic unit consisting of a pulser, a receiver, and a time counting circuit. The grease should allow for the ultrasonic wave generated by the transducer to propagate into the culvert wall. The electronic unit also has a numerical display which gives digital wall thickness readings. As the transducer is excited with a voltage pulse or spike, an ultrasonic pulse (main bang) is generated which traverses the bond, then enters the culvert wall, gets reflected on the opposite wall and returns to the transducer where a voltage is generated and the travel time is measured by the counting circuit in the electronic unit. The thickness of the culvert wall is then inferred by using the known sound velocity in the culvert using the fact that the velocity of the ultrasonic wave in the material times the travel time of the ultrasonic wave equals the distance traveled by the ultrasonic wave. The distance traversed by the ultrasonic wave in the material is twice the thickness of the material of interest.
Now the problems with the correct procedures are defined as follows:
The culvert wall thickness measurements must be taken in wet and dry conditions, but the grease which is used for bonding cannot be reliably applied and maintained under water.
When using contact transducers, the return time of the echo coming back from the opposite side of the culvert is so small, that part of it may fall within the recovery time of the main bang or voltage which signifies the initial excitation of the transducer. In this case the time measuring circuit produces erroneous readings, which the operator often cannot discern from valid ones.
The culvert often has an orange-peel like surface, which is a result of the corrosion that is taking place. The surface conditions preclude good bonding to a solid transducer and results in weak coupling and weak echoes, with ambiguous return times.
It is therefore difficult and impossible to obtain reliable readings above or below the water line when the culvert is corroded and in service using contact transducers.
Accordingly, it is an object of this invention to provide a method and system for measuring the wall thickness of culverts below and above their water line during the measurement process.
Another object of the invention is to provide a telemetry system which locates the position of the transducer when the thickness measurement is made allowing the record obtained at given time intervals to be compared and the required action taken, or measurement intervals determined.
Still another object of the invention is to provide a method in which a liquid delay is used to couple the sound to the culvert providing the operator a means for searching for a good echo.
Other objects will appear hereinafter.